CI: switch to GitHub Actions - step 1: sniff stage

This commit:
* Adds a GH Actions workflow for the CI checks which were previously run on Travis in the `sniff` stage.
    While these aren't 100% CS (= code style) checks, for the badge and workflow display, `CS` still seemed the most descriptive name.
*  Removes that part of the `.travis.yml` configuration.
* Adds a "Build Status" badge in the Readme to use the results from this particular GH Actions runs.

Notes:
Builds will run on all pushes and on pull requests, with the exception  of those just modifying files which are irrelevant to this workflow.

Author: jrfnl

.github/workflows/cs.yml

@@ -0,0 +1,66 @@
+name: CS
+
+on:
+  # Run on all pushes and on all pull requests.
+  # Prevent the build from running when there are only irrelevant changes.
+  push:
+    paths-ignore:
+      - '**.md'
+  pull_request:
+    paths-ignore:
+      - '**.md'
+
+jobs:
+  checkcs:
+    name: 'Basic CS and QA checks'
+    runs-on: ubuntu-latest
+
+    env:
+      XMLLINT_INDENT: '    '
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.4'
+          coverage: none
+
+      - name: 'Composer: adjust dependencies'
+        run: |
+          # The sniff stage doesn't run the unit tests, so no need for PHPUnit.
+          composer remove --no-update --dev phpunit/phpunit --no-scripts
+          # Using PHPCS `master` as an early detection system for bugs upstream.
+          composer require --no-update squizlabs/php_codesniffer:"dev-master"
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies
+        uses: "ramsey/composer-install@v1"
+
+      - name: Install xmllint
+        run: sudo apt-get install --no-install-recommends -y libxml2-utils
+
+      # Show XML violations inline in the file diff.
+      # @link https://github.com/marketplace/actions/xmllint-problem-matcher
+      - uses: korelstar/xmllint-problem-matcher@v1
+
+      # Validate the XML file.
+      # @link http://xmlsoft.org/xmllint.html
+      - name: Validate ruleset against schema
+        run: xmllint --noout --schema vendor/squizlabs/php_codesniffer/phpcs.xsd PHPCSDebug/ruleset.xml
+
+      # Check the code-style consistency of the XML file.
+      - name: Check XML code style
+        run: diff -B ./PHPCSDebug/ruleset.xml <(xmllint --format "./PHPCSDebug/ruleset.xml")
+
+      # Check the code-style consistency of the PHP files.
+      - name: Check PHP code style
+        run: composer check-cs
+
+      # Validate the composer.json file.
+      # @link https://getcomposer.org/doc/03-cli.md#validate
+      - name: Validate Composer installation
+        run: composer validate --no-check-all --strict

.travis.yml

@@ -29,7 +29,6 @@ env:
 # Note: for pull requests, "develop" is the base branch name.
 # See: https://docs.travis-ci.com/user/conditions-v1
 stages:
-  - name: sniff
   - name: quicktest
     if: type = push AND branch NOT IN (stable, develop)
   - name: test
@@ -38,39 +37,6 @@ stages:
 jobs:
   fast_finish: true
   include:
-    #### SNIFF STAGE ####
-    - stage: sniff
-      php: 7.4
-      addons:
-        apt:
-          packages:
-            - libxml2-utils
-
-      before_install:
-        - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
-        - export XMLLINT_INDENT="    "
-        # Set up CS check.
-        # - Using PHPCS `master` as an early detection system for bugs upstream.
-        # - The sniff stage doesn't run the unit tests, so no need for PHPUnit.
-        - travis_retry composer remove --dev phpunit/phpunit --no-update --no-scripts
-        - travis_retry composer require --no-update squizlabs/php_codesniffer:"dev-master"
-        - travis_retry composer install --no-interaction
-
-      script:
-        # Check the code style of the code base.
-        - composer check-cs
-
-        # Validate the xml file.
-        # @link http://xmlsoft.org/xmllint.html
-        - xmllint --noout --schema ./vendor/squizlabs/php_codesniffer/phpcs.xsd ./PHPCSDebug/ruleset.xml
-
-        # Check the code-style consistency of the xml files.
-        - diff -B ./PHPCSDebug/ruleset.xml <(xmllint --format "./PHPCSDebug/ruleset.xml")
-
-        # Validate the composer.json file.
-        # @link https://getcomposer.org/doc/03-cli.md#validate
-        - composer validate --no-check-all --strict
-
     #### QUICK TEST STAGE ####
     # This is a much quicker test which only runs the unit tests and linting against the low/high
     # supported PHP/PHPCS combinations.

README.md

@@ -10,6 +10,7 @@ PHPCSDevTools for developers of PHP_CodeSniffer sniffs
 [![Last Commit to Unstable](https://img.shields.io/github/last-commit/PHPCSStandards/PHPCSDevTools/develop.svg)](https://github.com/PHPCSStandards/PHPCSDevTools/commits/develop)
 
 [![Minimum PHP Version](https://img.shields.io/packagist/php-v/phpcsstandards/phpcsdevtools.svg?maxAge=3600)](https://packagist.org/packages/phpcsstandards/phpcsdevtools)
+[![Build Status CS](https://github.com/PHPCSStandards/PHPCSDevTools/workflows/CS/badge.svg?branch=develop)](https://github.com/PHPCSStandards/PHPCSDevTools/actions)
 [![Tested on PHP 5.4 to nightly](https://img.shields.io/badge/tested%20on-PHP%205.4%20|%205.5%20|%205.6%20|%207.0%20|%207.1%20|%207.2%20|%207.3%20|%207.4%20|%20nightly-brightgreen.svg?maxAge=2419200)](https://travis-ci.com/PHPCSStandards/PHPCSDevTools)
 
 [![License: LGPLv3](https://poser.pugx.org/phpcsstandards/phpcsdevtools/license)](https://github.com/PHPCSStandards/PHPCSDevTools/blob/stable/LICENSE)